AUTHOR=Tseng Hsiao-Chun , Yuh Han Yokie Tai , Lin Chia-Chia , Gong Gwo-Ching TITLE=Seasonal variations of nitrous oxide in a populous urban estuary and its adjacent sea JOURNAL=Frontiers in Earth Science VOLUME=11 YEAR=2023 URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2023.1112192 DOI=10.3389/feart.2023.1112192 ISSN=2296-6463 ABSTRACT=

The first investigations of seasonal N₂O variations and water-to-air fluxes in the Tamsui River estuary and its adjacent sea were carried out in this study. In the Tamsui River estuary, the concentration of N₂O decreased with increasing salinity. The seasonal variations of N₂O concentrations in the estuary were 46.8–148.5 nM in autumn, 15.9–82.5 nM in spring, 11.0–42.0 nM in summer and 13.1–120.6 nM in winter. When salinity regressed to zero, N₂O concentration was highest in autumn, followed by winter, spring, and summer, which might be influenced by the DO and NO₃⁻ concentrations as well as temperature. Because of mountains occlusion, the seasonal variations in wind speed were not large in the Tamsui River estuary. Seasonal variations of N₂O fluxes in the estuary were 10.9–35.6 μmol m⁻² d⁻¹ in autumn, 2.8–15.1 μmol m⁻² d⁻¹ in spring, 2.4–9.5 μmol m⁻² d⁻¹ in summer and 2.7–26.8 μmol m⁻² d⁻¹ in winter. In the adjacent sea of Tamsui River estuary, seasonal average N₂O concentrations in the surface seawater were 10.3 ± 0.2 nM in autumn, 11.6 ± 1.2 nM in spring, 11.4 ± 0.7 nM in summer and 13.8 ± 0.9 nM in winter, with no significantly seasonal changes while wind speed varied greatly seasonally. Seasonal variations of average N₂O fluxes in Tamsui River estuary’s adjacent sea were 40.3 ± 0.7 μmol m⁻² d⁻¹ in autumn, 19.7 ± 2.1 μmol m⁻² d⁻¹ in spring, 20.9 ± 1.3 μmol m⁻² d⁻¹ in summer and 49.0 ± 3.3 μmol m⁻² d⁻¹ in winter. As a result, seasonal variations in N₂O fluxes in the estuary were dominated by N₂O concentrations in the water, whereas in the sea, it was dominated by wind speed. Overall, the Tamsui River estuary and its adjacent sea were net sources of atmospheric N₂O with annual average fluxes 10.6 ± 6.7 and 32.5 ± 14.5 μmol m⁻² d⁻¹, respectively.